Astronomy and Astrophysics – Astrophysics
Scientific paper
1999-11-23
Astronomy and Astrophysics
Astrophysics
13 pages, 7 figures, Accepted for publication in A&A
Scientific paper
Increasing evidences provided primarily by the cluster lensing and numerical simulations of cluster formation indicate that galaxy clusters may contain compact cores that are substantially smaller than the cores revealed by the X-ray observations of hot intracluster gas. In this paper we present a model that describes how two distinct cores can grow simultaneously as a result of infall from the background dark matter. This model needs a pre-existing localized large fluctuation, which can be the non-Gaussian density peak of the primordial fluctuations, to seed the individual growing cluster. On the other hand, numerous recent observations also show that no strong evolution is detected for galaxy clusters within the redshift up to $z\sim 0.8$. We therefore present a comparison of observations with the saturated cluster configuration resulting from our infall model. In the saturated state, the predicted compact core mass is about few$\times 10^{11} M_\odot$ and the core size about 1 kpc for a cluster mass $\sim$ several $\times 10^{15} M_\odot$ within a radius of 3 Mpc. We have successfully reproduced the dark matter distribution revealed by the gravitational lensing, and the observed radial distributions of cluster galaxies, intracluster gas (i.e., the conventional $\beta$ model) and baryon fractions in a consistent way. This model, when combined with the observed X-ray surface brightness profiles of clusters, predicts that the overall temperature of intracluster gas has a tendency of radial decline with a mean polytropic index $\gamma\approx1.2$. Finally, the so-called $\beta$ discrepancy finds a natural explanation for such a polytropic gas in our model.
Chiueh Tzihong
Wu Xiang-Ping
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